The present invention relates to a coating for metal components which is employed, in particular, as a piston coating for pistons in internal combustion engines, as well as to a process for coating a metal component. In addition, the invention relates to a piston for internal combustion engines and a motor vehicle having the piston according to the invention.
To increase the resistance and, in particular, for use in tribologically highly stressed applications, steel components are coated with an abrasion-resistant and friction-reducing layer to reduce wear and friction. DE 10 2010 047279 A1 discloses a coating for pistons for internal combustion engines, which consists of a wear-resistant inner layer composed of a polymer matrix with ceramic particles, aramid fibers and/or carbon fibers dispersed therein and an outer layer composed of a polymer matrix with solid lubricants dispersed therein. Disadvantages of this piston coating are the reduced adhesion to the piston surface and an insufficiently low coefficient of friction, in particular during the running-in time of the piston.
Proceeding from this prior art, it is therefore an object of the present invention to provide a coating for metal components and also a piston for internal combustion engines, which display a high abrasion resistance combined with a durably low coefficient of friction. In addition, it is an object of the invention to provide a motor vehicle which has a low fuel consumption and reduced CO2 emission and hydrocarbon emission. It is likewise an object of the invention to provide a coating process for metal components having high stability and good tribological properties, which process is easy to implement.
In the case of a coating for metal components, these and other objects are achieved according to the invention by a multilayer structure. The multilayer structure comprises an inner layer which acts as bonding layer and bonds directly to the metal component and can provide a good bond between the coating and the metal material. For this purpose, the inner layer comprises at least one layer which is selected from among a metal layer, a metal carbide layer, a metal nitride layer, a metal carbonitride layer and a metal-containing hydrocarbon layer, and can also comprise combinations of individual layers of these types. The metals in the respective individual layers of the inner layer can comprise one metal or be used in doped form. Particularly suitable doping elements are selected from the group consisting of boron (B), nitrogen (N), silicon (Si), oxygen (O), fluorine (F), phosphorus (P), tantalum (Ta), molybdenum (Mo), nickel (Ni), chromium (Cr), gold (Au), niobium (Nb), tungsten (W), copper (Cu) and titanium (Ti), and can be selected according to the desired property to be achieved. It is also possible to employ combinations of a plurality of doping elements. Typical contents of the doping elements are in the range from 5 to 50 at %, preferably from 20 to 50 at %, in each case based on the total content of the respective metal in the individual layer. The proportion of dopant in the inner layer is preferably gradated and decreases in the direction of the metal component to be coated. All individual layers of the inner layer have a metal content which can form a good bond with the metal surface and thus achieve good adhesion of the coating.
An intermediate layer which comprises at least one layer of amorphous carbon and thereby introduces friction-reducing properties into the coating is applied on top of the inner layer. The layer of amorphous carbon is not subject to any detailed restrictions and can be selected according to the desired physical, mechanical and chemical resistance. Suitable amorphous carbon layers are known, for example, from the VDI Guideline 2840.
A covering layer applied on top of the intermediate layer forms a surface layer which is in contact with the surroundings of the metal component. According to the invention, the covering layer comprises a W—C:H layer or an a-C:H* layer. In other words, a W—C:H layer or an a-C:H* layer thus forms an outermost layer of the coating of the invention, which is in direct contact with the surroundings of a metal component to be coated.
For the purposes of the invention, a W—C:H layer is a hydrocarbon-containing tungsten layer. An a-C:H* layer is a gradated layer of amorphous carbon which is composed of diamond-like parts (sp3 hybridization) and graphite-like parts (sp2 hybridization), where the proportion of graphite-like parts is greater than in an a-C:H layer (DCL—diamond-like carbon) and is, in particular, from 20 to 50 at % and a proportion of diamond-like parts is from 10 to 30 at %. The hardness increases in the direction of the metal component, i.e. in the direction of the inner layer, so that the sp2 proportion decreases in the direction of the inner layer and the sp3 proportion increases. The a-C:H* layer can be doped with nitrogen or hydrogen or another doping element in order to decrease the hardness. Typical contents of the doping elements are in the range from 5 to 50 at %, preferably from 20 to 50 at %. The proportion of dopant in the covering layer is preferably gradated and decreases in the direction of the metal component to be coated. A particularly advantageous effect can be achieved thereby: when the covering layer is subjected to friction, solid lubricant particles are formed and bring about an additional reduction in the coefficient of friction of the coating of the invention. Thus, for example, when the coating is correctly used, tungsten sulfide is formed from sulfur from a lubricating oil and the W—C:H covering layer and graphite is generated from the a-C:H* layer. A high sp2 hybridization content, i.e. an at least higher proportion of sp2 parts, as is present in a conventional a-C:H layer, is therefore advantageous for the formation of graphite. The formation of the solid lubricant directly from the covering layer has the advantage over particulate, solid lubricants, for example those present in a polymer composite, that they are formed selectively where abrasion of the covering layer has occurred, so that the sliding properties of the coating remain equally good over a long time and the stability of the remaining layer composite is not impaired.
However, the effects according to the invention can be achieved only when the layer composite is intrinsically stable, abrasion-resistant and friction-reducing. This is achieved by a maximum layer thickness of the coating being not more than 5 μm. Owing to the high abrasion resistance and very good tribological properties of the coating according to the invention, the coating is particularly suitable as coating for pistons for use in internal combustion engines. Even under the very high frictional stresses and high temperatures occurring there, the coating is stable and displays durable low coefficients of friction. Even when the coating has been partly abraded away, the friction of the piston in a slideway surrounding it (cylinder interior) is reduced sufficiently since further solid lubricant is formed from the covering layer at any time by operation of the piston. Thus, the piston/slideway system can run in more quickly, which is particularly advantageous with a view to engine start-stop automation, which goes through a critical rotational speed during starting.
An advantageous layer structure of the inner layer provides a Cr layer (chromium layer), a Cr/WC layer (chromium/tungsten carbide layer) and a W—C:H layer or a CrN layer (chromium nitride layer), a Cr/WC layer and a W—C:H layer or a Cr layer, a CrN layer and a Cr/WC layer, each of which are arranged or applied on top of one another in this order. The Cr layer or the CrN layer thus forms the lowermost layer of the inner layer. Instead of the W—C:H layer, it is also possible to use a WC/W—C:H layer. The Cr layer or the CrN layer is intended to be applied to the metal component. Owing to their metallic character, these layers are particularly well suited for forming a stable, abrasion-resistant bond to metal materials and in particular to steels or aluminum alloys and can be produced at acceptable prices. The Cr/WC layer serves as support layer and ensures good stability within the layer composite in the inner layer. Due to its proportion of Cr, it can form a mechanically particularly stable and abrasion-resistant bond to the Cr layer or CrN layer joined to it. On the other hand, the tungsten content ensures good bonding to the W—C:H layer, which additionally makes a contribution to minimizing friction of the coating of the invention.
With a view to reducing the coefficient of friction of the coating for metal components, the intermediate layer also advantageously comprises an a-C:H layer. An a-C:H layer is an amorphous carbon layer (also known as DLC (diamond-like carbon)), which has an sp3 proportion of at least 20 at %. The a-C:H layer can be doped, for example with hydrogen (H), Si, N, P or oxygen. Typical contents of the doping elements are in the range from 5 to 50 at %, preferably from 20 to 50 at %. The proportion of dopant in the intermediate layer is preferably gradated and decreases in the direction of the metal component to be coated.
A further advantageous embodiment provides for the intermediate layer to comprise an a-C:H layer and an a-C:H* layer, where the a-C:H layer is arranged facing the inner layer and the a-C:H* layer is arranged facing the covering layer. Due to the higher proportion of sp2 parts in the a-C:H* layer compared to the a-C:H layer, the hardness of the intermediate layer decreases in the direction of the covering layer. This leads, in combination with the covering layer, to a soft top layer which has good running-in properties and displays very good coefficients of friction. In the direction of the inner layer, the a-C:H layer ensures, due to its higher hardness, good stability of the layer structure and very good adhesion in the layer composite.
In a particularly advantageous embodiment, the coating for metal components is characterized in that the inner layer is formed by a Cr layer, a Cr/WC layer and a W—C:H layer, and the layers are applied in this order or layer sequence to a metal component. The Cr layer is bound directly to the metal component, i.e. applied to the metal component. According to this embodiment, the intermediate layer is an a-C:H layer and the covering layer is a W—C:H layer or an a-C:H* layer. As an alternative, the coating of the invention for metal components is characterized in that the inner layer is formed by a Cr layer, a Cr/WC layer and a WC/W—C:H layer in this order, where the Cr layer is intended to be applied to the metal component. The intermediate layer additionally comprises an a-C:H layer facing the inner layer and an a-C:H* layer facing the covering layer. A W—C:H layer is provided as covering layer. These coatings have a very balanced property spectrum in respect of mechanical stability, abrasion resistance and very good tribological properties and are particularly well suited as coating for pistons for internal combustion engines.
The invention likewise describes a piston for internal combustion engines which displays a high abrasion resistance and very good sliding properties combined with very good frictional and wear behavior. The piston is a metal component pursuant to the invention, for example made of aluminum, and has a coating as described above. The coating can be applied to part of the piston surface or else completely surround the piston sliding surface, known as the piston skirt. Compared to a merely partial coating, this has the advantage that the piston is coated around the entire circumference of its surfaces which are in contact with a piston slideway and is thus protected against frictional wear. The piston of the invention displays quiet running behavior, a low wear rate and high abrasion resistance combined with a high operational range, by way of which fuel consumption can be reduced and CO2 emission and hydrocarbon emission can thus be reduced or minimized.
Furthermore, the invention also describes a motor vehicle which has at least one piston as described above. The motor vehicle is characterized by high ride comfort, low fuel consumption and low CO2 emission.
The invention additionally provides a process for coating metal components. The coating is configured as multilayer structure. The process comprises the action steps of:                applying an inner layer comprising at least one layer selected from among: a metal layer, a metal carbide layer, a metal nitride layer, a metal carbonitride layer, a metal-containing hydrocarbon layer and combinations thereof; to a metal component,        applying an intermediate layer on top of the inner layer, where the intermediate layer comprises at least one layer of amorphous carbon, and        applying a covering layer which comprises a W—C:H layer or an a-C:H* layer.        
The multilayer structure is configured so that a maximum layer thickness of the coating is not more than 5 μm. As regards the definitions of the individual layers, reference is made to what has been said in respect of the coating according to the invention for metal components. The process is easy to implement inexpensively by application of the respective individual layers on top of one another using standard processes without a great engineering outlay and makes it possible to produce an abrasion-resistant and mechanically stable coating having very good sliding properties and thus a low coefficient of friction. The process is particularly suitable for the production of coatings for pistons which are employed in internal combustion engines.
The advantages, advantageous effects and embodiments described for the coating of the invention also apply to the piston of the invention, the motor vehicle of the invention and the coating process of the invention for metal components.
One advantageous embodiment of the process of the invention provides for the application of the layers to be carried out by use of physical or chemical vapor deposition, in particular by PVD or PECVD. The processes described here are easy to implement and allow the formation of layers having a precise layer thickness while avoiding defect structures.
To increase the abrasion resistance of the coating on the metal surface, a Cr layer, a Cr/WC layer and a W—C:H layer or a CrN layer, a Cr/WC layer and a W—C:H layer or a Cr layer, a CrN layer and a Cr/WC layer are deposited in succession in order to apply the inner layer to the metal component.
The solutions according to the invention and the further developments thereof provide the following advantages:                The coating is abrasion-resistant, mechanically stable and thus withstands high friction forces.        The coating has very good tribological properties, i.e. very good sliding properties combined with a low coefficient of friction, and is therefore particularly suitable for tribologically highly stressed applications.        The piston has a durably quiet running behavior, good abrasion resistance and low coefficients of friction in the surrounding piston slideway.        The motor vehicle has a long operational range combined with a low fuel consumption, low CO2 emission rates and low hydrocarbon emission rates.        The coating process can be implemented easily and thus inexpensively without a great engineering outlay by combination of standard processes.        
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.